In any optical element, a portion of the incident light is reflected at each surface. The exact amount is determined by the refractive index change at the dielectric interface. The amount of light so reflected can be reduced by means of anti-reflection coating. There are four main reasons why anti-reflection coatings are desirable in optical systems. First, the percentage of reflected light is lowest at normal incidence. The percentage increases with a corresponding increase in the angle of observation. This limits the resolution of the image and, in some cases, can completely obscure the image. Second, an increase in reflected light corresponds to a decrease in transmitted light. For components such as compound lenses, this loss is multiplicative and may be intolerable. Third, reflections from optical surfaces often create unwanted or distracting glare. Finally, for components such as camera lenses and photographs, that contain many optical surfaces, there are multiple internal reflections. These reflections can cause stray light to hit the image plane and thereby reduce the image contrast and definition.
It has been well known for many years that unwanted reflections can be substantially reduced by providing a surface coating of an optically clear coating material having a refractive index which is lower than the refractive index of the substrate. However, difficulty in producing high quality thin films prevented significant practical application until approximately 1940, when technology for applying thin films of certain refractory inorganic materials via evaporation under high vacuum conditions was developed. More recently, low refractive index polymeric coatings, generally fluoropolymer coatings, have been developed for anti-reflection applications. Generally, these coatings, for maximum effectiveness, are about 1/4 wavelength thick. The basic theory of such anti-reflection coatings is well known; the technical challenge is in the provision of conveniently applied, effective, strongly adherent, scratch-resistant and relatively low cost coatings with optimally low refractive index, over large areas.